Stimulation of human leukocytes in vitro with the mitogen, staphylococcal enterotoxin A, results in highly variable quantities of interferon. Of the cultures tested, approximately 30% did not produce detectable gamma- or immune interferon, and about 30% synthesized significant quantities. Messenger RNA isolated from the cells of selected donors gave rise to functional gamma-interferon when translated in Xenopus laevis oocytes. The mRNA coding for this protein is the largest interferon mRNA so far encountered in human cells, sedimenting at 18S in formamide-sucrose density gradients. Relative to the cytoplasmic polyadenylated RNA obtained from the entire leukocyte population, the level of gamma-interferon mRNA on a molecular basis was approximately ten parts per million.